This invention relates to a microwave tube such as a klystron or a travelling wave tube, and more particularly a microwave tube provided with a permanent magnet type magnetic circuit for focusing an electron beam.
The prior art microwave tube of this type comprises an electron gun for emitting an electron beam, a high frequency circuit unit for causing an interaction between the electron beam and an input high frequency wave, a hollow cylindrical collector for collecting the electron beam after the interaction which is aligned in the axial direction, and a magnetic circuit including a permanent magnet for producing an axial flux in the high frequency circuit unit for focusing the electron beam. The electron beam emitted by the electron gun contributes to amplifying the input high frequency wave while travelling along the axis of the high frequency circuit unit in a state focused by the magnetic flux so as to give a part of the beam energy to the high frequency wave and is finally collected by the collector.
With the prior art construction described above, however, as the density of the axial flux produced by the magnetic circuit is low at a point near the entrance of the collector, focusing the electron beam is not enough so that the electron beam will be spread by space charge. As a consequence, substantial members of electrons collide against the collector near its entrance. Since the portions of the collector near its entrance are formed in a thin walled nozzle shape with small thermal capacity and small heat conductivity by taking into consideration the arrangement of the magnetic circuit, such portions will be overheated by the collision of the electrons thus evolving gas. In an extreme case these portions melt and come to contact with the magnetic pole piece comprising the magnetic circuit.
Of course, such overheating can be more or less alleviated by increasing the thickness of the portions of the collector near its entrance. However, when the thickness is increased toward inside, in other words, when the inner diameter of the collector is decreased, the secondary electrons generated by the collision of the electrons tend to move toward the high frequency circuit unit thus causing various troubles. On the other hand, when the thickness is increased outwardly, the outer diameter of the collector tip increases thus requiring to increase the outer diameter of the magnetic pole piece which is constructed similar to the collector and located near the collector tip. This increases the size of the focusing magnetic circuit as well as the weight and price. Especially, microwave tubes operating at a relatively high frequency and high power, require to have high flux density along the axis. For this reason, where it is necessary to increase the thickness of the pole piece, collector thickness also increases thereby increasing the size and cost of the magnetic circuit. As has been pointed out hereinabove, increase in the wall thickness at the collector entrance increases the number of electrons colliding upon the inner surface of the entrance thus increasing heat generation and moving back secondary electrons, and various expedients for preventing such overheating may be accompanied by a lot of sacrifice.